1. Field of the Invention
The present invention relates to a method for designing the length of a polarization maintaining fiber provided at the output side of a laser diode module for outputting laser light to be depolarized, a method for designing the length of a depolarizer, a laser diode module and a depolarized laser diode module manufactured based on the design method, and an optical amplifier which utilizes such a laser diode module as a pump light source.
2. Related Art
An optical amplifier for amplifying signal light in an optical fiber by utilizing as pumping means laser light from a semiconductor laser device (laser diode: LD) plays an important role in a optical communication system. Among such optical amplifiers, an optical fiber Raman amplifier, which utilizes a Stimulated Raman Scattering in an optical fiber, has outstanding noise performance. Therefore, an optical fiber Raman amplifier has become indispensable for optical amplifying means in a long-distance transmission system, and for speed enhancement of a transmission signal.
Since optical Raman amplification uses the Stimulated Raman Scattering process occurring in an optical fiber, the size of a Raman gain for signal light largely depends on cross relationship between the state of polarization of the signal light and that of pump light. For example, where the state of polarization of pump light is linear polarization (usually, the polarization of laser light output from an LD is approximately linear polarization), when the state of polarization of the signal light is linear in the same direction as that of the pump light, the Raman gain becomes high, and when the direction of polarization of the signal light is orthogonal to that of the pump light, the Raman gain becomes reduced. In other words, there is Polarization Dependence of Gain (Polarization Dependent Gain: PDG) such that the amount of a gain depends on mutual relationship between the state of polarization of signal light and that of pump light.
In order to diminish the polarization dependent gain of Raman amplification, pump laser light is preferably laser light of which the state of polarization is changed at an extremely short time interval. As such laser light, one is such as obtained by polarization beam combining of two LDs of the same wavelength and the other is such as depolarized by a depolarizer arranged at an output side of an LD. Both of them present small DOP values (Degree Of Polarization) indicative of the percentage of a polarized component and their states of polarization are both change at extremely short time intervals.
Used as a depolarizer for depolarizing laser light output from an LD are birefringent crystals and birefringent optical fibers. A DOP value depends on the length of these depolarizers (distance in the direction of light travel), and it is known that basically, the longer a depolarizer is, the smaller the DOP value becomes and that the DOP value also depends on an FWHM (full width half maximum) of laser light spectrum output from an LD. For this reason, conventionally, the following method was applied to reduce a DOP value. First, the length of a depolarizer was roughly determined on the basis of conventional design information in the light of qualitative relationship between an FWHM of an LD and the DOP, then the depolarizer of determined length was coupled to a polarization maintaining fiber provided at the output side of a laser diode module, and the fiber length was adjusted while actually measuring DOP values.
However, with the above-mentioned method, it was required to fine-adjust the length of a depolarizer while measuring the DOP values of output laser light, which produced a problem that fine adjustment took a lot of time and troubles and increased manufacturing cost. In addition, the DOP was actually decreased not uniformly over the length of the depolarizer and took a maximum value periodically, and therefore, in order to evade the maximum value, it was required to fine-adjust the length of the depolarizer while measuring the DOP values.
Further, even if the length of a depolarizer was optimized with use of the above-described method, since a DOP value varied corresponding to fluctuation of the ambient temperature of a depolarized laser diode module, fine adjustment was hard to realize, and so was a small value of DOP (e.g., 10% or less). Furthermore, the fluctuation of DOP caused fluctuation in Raman gain, which necessitates controlling of the ambient temperature of the depolarized laser diode module.